The present invention relates to protection circuits for power switches, and in particular, to a reverse battery protection circuit for a power switch.
Power switches are employed to switch electrical loads, for example, such loads as electrical motors. With reference to FIG. 1, a simplified prior art circuit showing a driver 30 driving a power switch 10 comprising a MOSFET is shown. The MOSFET switches the load 20, which might comprise an electrical motor. In an automotive application, the load and power switch are provided in series across the battery connections. The MOSFET power switch 10 includes a body diode 10A, which, in normal operation when the battery is connected as shown in FIG. 1, is back biased. However, as shown in FIG. 2, if the battery connections are inadvertently reversed, body diode 10A will conduct, and because of its relatively high forward voltage drop, dissipate a substantial amount of power which can lead to destruction of the switching device 10. For example, as shown in FIG. 1, when the battery is connected properly, the power dissipation in the semicondutor switch 10 is on the order of approximately 1 watt, assuming a current through the load of about 10 amps and an RDSon of the semiconductor switch of about 10 milli ohms. If the battery is improperly connected in a reverse battery connection, which may occur accidently upon improper installation of the battery or during a reverse jumping of a dead battery from a good battery, the power dissipated in the body diode of the switch, which will be forward biased as shown in FIG. 2, is approximately 6 watts considering that the forward voltage drop of the diode is 0.6 volts and the current remains at 10 amps. This six-fold power dissipation may destroy the switching device.